Human stem cells hold great promise for the treatment of a variety of diseases. However, their use as a therapy is hampered by limited information on the physiologic regulation of homing and engraftment of the cells in the microenvironmental niche. In the stem cell niche, neighbouring subsets of cells and extracellular substrates house stem cells and provide specialized functions to modulate stem cell self renewal and progeny production in vivo. Using a transgenic mouse model, we previously demonstrated that osteoblasts are a key constituent of the murine HSC niche in vivo and that increased number and activity of the osteoblasts leads to an increase in the number of HSCs. We also demonstrated that stem cell localization at the murine endosteal niche is dependent upon the calcium sensing receptor. In this proposal we propose to examine human stem cell interactions within a human bone marrow microenvironment. This will be achieved through the use of murine model systems that have a human fetal bone explant. These models are in contrast to mouse models where the human stem cells engraft in a murine bone marrow microenvironment. This will be achieved through two specific aims. Firstly, we will investigate the location of the human HSC niche in vivo. Secondly, using a candidate approach from knowledge gained in our murine model systems, we will identify the mechanisms for human HSC engraftment in the niche in vivo. By using the SCID-hu model we anticipate to be able to identify the mechanisms responsible for human stem cell engraftment in their respective microevironmental niche. These studies will provide insight into the dynamics of human HSC engraftment and self-renewal in vivo by providing information regarding the exact mechanism of HSC homing to and location of the human bone marrow niche. Understanding the biology of human stem cell engraftment and self-renewal offers a broad-spectrum of therapeutic potential for various hematologic and oncologic diseases, the ultimate goal of these studies. [unreadable] [unreadable] [unreadable]